The use of communication cables which include a plurality of optical fibers is rapidly expanding. An optical fiber cable may comprise a plurality of glass fibers each of which is protected by at least one layer of a coating material. The optical fibers may be assembled into units in which the fibers are held together by binder ribbons to provide a core. In one manufacturer's line of cables, the core is enclosed by a plastic tube and a plastic jacket.
Whatever the structure of a cable, there must be provisions for connecting, such as by splicing, transmission media at an end of a given length of cable to corresponding transmission media at an adjacent end of another length of cable. It is conventional to use a closure, within which all conductors are connected, wrapped and stored and protected environmentally.
During the connection of metallic conductors, it is customary to bend sharply the conductors, to provide access to other connections. The physical nature of glass optical fibers forecloses the adoption of connectorization techniques which are used with metallic conductors within such a closure. Because of their small size and relative fragility, special considerations must be given to the handling of optical fibers in closures. Transmission capabilities may be impaired if an optical fiber is bent beyond an allowable bending radius, the point at which light no longer is totally contained in the core of the fiber. Furthermore, expected lives of the fibers will be reduced if bent to less than the minimum bending radius. Generally, the radius to which the optical fiber can be bent without affecting orderly transmission is substantially greater than that radius at which the optical fiber will break. Whereas glass and silica, the materials used to make optical fibers, are in some respects stronger than steel, optical fibers normally do not possess this potential strength because of microscopic surface fractures, which are vulnerable to stress and spread, causing the fiber to break easily.
It should be clear that, an optical fiber cable does not lend itself to connecting practices of wire-like communication conductors. The individual glass fibers cannot just be twisted, tied, wrapped and moved into a closure, in anything like the manner of wire-like metallic conductor cables. These small diameter glass fibers cannot be crimped or bent at small angles, without breakage. Inasmuch as glass fibers have memory and tend to return to a straight-line orientation, placement in a closure becomes somewhat difficult. Moreover, the interconnection of optical fibers is a precision operation which is somewhat difficult to perform within a manhole, or at pole-suspension elevation, for example. What is needed is a closure in which connected optical fibers have at least a minimum bend radius or, preferably, no substantial curvature.
Also, there is a need for a closure which is particularly suitable for the fiber-in-the-loop market and to splice relatively small count optical fiber cables some of which are referred to as drop cables. Drop cables are those cables which extend from distribution cables at the street to the premises. For such a use, what is sought after is a closure that is relatively inexpensive to serve this potentially very large market. Also, desirably, the sought after closure is relatively small in size yet able to accommodate a suitable number of splices for fiber-in-the-loop.
As might be expected, fiber closures are available in the prior art. Some of these prior art closures have shortcomings insofar as being used in the fiber-in-the-loop market.
In the prior art, fiber slack normally has been provided adjacent to connective arrangements. When splicing optical fibers by mechanical means or by fusion, it becomes necessary to provide enough slack fiber so that the fiber can be pulled out of a closure and positioned in apparatus for the preparation of fiber ends and the joining together. This typically has required that at least about 0.5 meter of fiber from each cable be stored in the splice closure when the closure is sealed, that is when the splicing has been completed. There must be a method of storing this slack which usually requires inducing curvature no less than the minimum bend radius of about 3.7 to 5 cm in the fibers, of protecting the splices and of keeping the fibers together in an orderly manner. The need to store the slack further complicates the problem of providing a suitable optical fiber closure. Some prior art closures have included organizers which have tended to place higher than desired stresses on the optical fibers, resulting in fiber damage.
In another closure, a tubular cover having a closed end and an open end is adapted to receive and be sealed to a cable termination assembly. The cable termination assembly includes cable entry facilities through which the cables to be spliced are routed. A support member extends from the cable entry facilities and has a free end disposed adjacent to the closed end of the cover. The support member includes a support base for supporting an optical fiber breakout and a plurality of optical fiber splice trays.
Mounted centrally of each tray is at least one organizing module each of which is capable of holding a plurality of optical fiber connective arrangements. Each module is such that it is capable of accommodating different kinds of connective arrangements such as, for example, fusion splices and mechanical splices, both polished and non-polished. Each tray is capable of holding a plurality of organizing modules which may be added as needed. Although this last-described closure has enhanced storage capability both in number and in kind, which is ideal for high density applications, it is larger and has more storage capability than is needed for some applications in fiber-in-the-loop and for splicing small fiber count cables.
Further, the sought-after closure should be suitable for repair operations to optical fiber drops in fiber-to-the-home installations, for example. Typically, in such installations, an optical fiber drop cable which has been cut or damaged must be repaired. One alternative is to run another length of optical fiber cable from the street to the premises. Another approach would be to remove a portion of the damaged cable on each side of the break and to splice in a new, short length. The latter approach only becomes feasible if closures used at each end of the new, short length for splicing to the priorly installed cable are economical in cost.
The prior art includes at least one drop repair closure. See U.S. Pat. No. 4,820,007 which issued on Apr. 11, 1989 in the names of R. R. Ross and I. Vedejs. In it, a splice tray includes provisions on one side for holding optical fiber splices and metallic conductor splices on an opposite side. An electrical bonding and gripping assembly is adapted to be mounted on the splice tray. The closure also includes mating cover portions which are moved into engagement with each other to enclose the tray. Also, a waterblocking encapsulant may be introduced into the closure.
The last described prior art drop repair closure is designed primarily for mechanical and fusion splices which require long lengths of fiber slack for splicing and bend radius controllers. Because of the amount of slack, a fiber organizer and splice holder are required for storage of slack fiber. Also, it has a relatively large size compared to the transverse cross section of cables which are spliced therein. As such, it is more vulnerable than desired to repeated engagement by excavation tools. Further, because of its relatively large size compared to the cables to be spliced therein, the amount of encapsulant necessary to fill it is relatively large. Gripping of the cables at the ends of the closure is accomplished with the same assembly which is used for bonding. Lastly, the only effective barrier to moisture ingress is the encapsulant.
What is sought-after and seemingly what does not appear in the prior art is a closure which is relatively small and what is suitable for drop cable repair installations. The sought after closure must include facilities for carrying electrical continuity and cable strength across connective arrangements and, of course, must not introduce excessive bending into the optical fiber.